1. Field of the Invention
Example aspects of the invention relate generally to the manufacturing of electronics products, and, more specifically, to the processing of electronics components where liquid flow control and containment is required.
2. Description of the Related Art
Packaging for power electronics can require the use of metallized substrates that provide electrical isolation between components yet maintain high electrical and thermal conductivity. Such components, which include diodes, metal oxide semiconductor field-effect transistors (MOSFETs), insulated-gate bipolar transistors (IGBTs), resistors, and capacitors, are typically attached to substrates using solder alloys. During the attachment process, however, the flow of solder must be controlled to achieve the necessary electrical isolation between components and to maintain the proper orientation and alignment of the components. A known practice is to apply a polymer onto a substrate for stopping and controlling solder flow. Industry standard polymers include epoxies and polyimides; particular polymers include Taiyo PSR 4000, Taiyo PSR AUS5, Hitachi HP 300, and Epotek TV1002.
Known solder barrier materials, however, suffer drawbacks. While application of these materials often is straightforward, material degradation can subsequently occur. Polymers such as epoxy have a low glass transition temperature—typically around 125° C.—beyond which the material can soften and electrically degrade. On the other hand, polyimide exhibits inherently poor adhesion to most metallized surfaces. Its adhesion reduces further at high temperatures or when subjected to multiple temperature cycles, as may occur during manufacturing and application. Accordingly, epoxy—and polyimide—based polymers are susceptible to delamination and/or scaling. Traditionally, solder reflow occurred at a furnace peak temperature of 230° C. Recently, however, the European Union's Restriction of Hazardous Substances (RoHS) Directive has forced the global electronic packaging industry to switch to the use of lead-free solder alloys. These alloys typically reflow at furnace peak temperatures of at least 250° C.; some reflow as high as 320° C. These more-demanding solder reflow environments can lead to or exacerbate delamination and scaling of polymer materials, diminishing their effectiveness as solder stops during reflow.
Polymer solder barrier materials can further degrade after solder reflow when used in high temperature environments. High-temperature storage and operating conditions are commonly stipulated as design requirements for state-of-the-art power electronics. While traditional power electronics are designed to operate at a maximum of 85° C., next-generation devices are required to operate at temperatures up to 250° C. Degradation, delamination and scaling of solder stop materials in a manufactured device can result in loose particles within the package. In accordance with military and other high-reliability standards, such as Military Performance Specification 19500 (MIL-PRF-19500), these particles are considered foreign object debris (FOD), which can compromise the long-term reliability of a device.
A further drawback of using a polymer material as a solder stop is that it increases the cost and lead time of a manufactured device. When used as solder stops, polymers are deposited and patterned in separate processing steps, which leads to additional processing time. Use of such polymers also requires additional material handling and expenditure as these special material formulations often are not used in any other device processing steps.
Thus, there is a need for a solder barrier that is physically stable and can be easily integrated into the manufacturing of high-temperature power electronics. This need also arises in other manufacturing processes that involve electronics components. Specific examples of such processes include joint brazing and liquid epoxy attach. Generally speaking, there is a need for a physical liquid barrier that can effectively control the flow and containment of liquids during electronic component processing, yet remain stable when subjected to high temperature processing and application environments.